Proinsulin Folding-Trafficking Relationship in Diabetes

Background: Diabetes is a metabolic disorder that has affected millions of people worldwide for centuries. The term "diabetes" was first used by Apollonius of Memphis, and "mellitus" refers to its sweet taste ^{1, 2}. While diabetes can be found in all countries, it is particularly prevalent in low and middle-income nations ³. The number of people with diabetes has been steadily increasing over the years ⁴. The age-adjusted prevalence has risen from 10.3% in 2001-2004 to 13.2% in 2017-2020 ⁴. This trend is alarming, as diabetes is a significant cause of death and disability³. In 2021 alone, diabetes was responsible for 6.7 million deaths globally ⁴. The World Health Organization (WHO) predicts that it will be the 7th leading cause of death by 2030 ⁵. Diabetes features high blood sugar levels, which happens when a subset of specialized pancreatic cells (called beta cells) does not produce enough insulin or the body doesn't use it properly ⁶. Insulin helps our body turn food into energy. There are two main types of diabetes: Type 1- Usually diagnosed in children, is caused by the body's immune system attacking insulin-producing cells, and Type 2- The most common type, occurs when the body becomes resistant to insulin or doesn't produce enough ⁷. Where does the insulin (in the beta cells) come from? It originates from a protein called “proinsulin”. The summary article below explains the connections between proinsulin, insulin, beta cell function, and diabetes.

Summary of the article “Proinsulin folding and trafficking defects trigger a common pathological disturbance of endoplasmic reticulum homeostasis” (Protein Sci. 2024 Apr; 33(4): e4949)

Imagine your body as a busy factory. Inside this factory, there are countless tiny workers, each with a specific job to do. One important job is to make a protein called proinsulin, and the beta cells in the pancreas are the workers responsible for this. Proinsulin is the raw material that our body uses to create insulin, a hormone that helps regulate your blood sugar levels. Proinsulin must “fold” into a certain shape or three-dimensional structure to make the insulin hormone ⁸.
When the Beta-Cells Go Wrong: Sometimes, the workers/factory can make mistakes. Instead of folding proinsulin correctly, it might do it wrong (“misfolding”). When misfolding happens, it can cause problems in a part of the cell called the endoplasmic reticulum (ER).
The ER: The Quality Control Department. The ER is like the quality control department of the cells including the beta cells. It checks to make sure that the proinsulin is built or folded correctly before it's shipped off to the immediate next compartment of the cell called Golgi. This process is called “ER-to-Golgi trafficking”. The beta cell ER is sometimes unable to fold proinsulin to its proper shape and that is detrimental to insulin making. Also, if too many incorrectly folded proinsulins pile up, it can stress the ER ⁹.
The Link to Diabetes: Proinsulin misfolding in the ER can also lead to problems with insulin production ¹⁰ and diabetes. How does this happen? Successful proinsulin trafficking from ER-to-Golgi is critical because right after this step, proinsulin faces enzymes that help convert proinsulin to the mature insulin hormone. Insulin is stored temporarily in tiny ball-like structures (called “secretory granules”) inside cells and secreted outside in response to high glucose. All of this will happen only if proinsulin folds correctly in the ER. As opposed to this, if proinsulin misfolds, it stays back in the ER directly hindering ER-to-Golgi trafficking, thereby preventing proinsulin-to-insulin conversion. This eventually would result in high blood sugar and diabetes. It was not long ago since proinsulin misfolding was reported in the early stages of type-2 diabetes ⁹. What is the fate of the misfolded proinsulin floating around inside the ER?

Normally, proinsulin (the precursor to insulin) has special chemical links called disulfide bonds that help it fold into the correct shape. But when proinsulin is made incorrectly (misfolded), these links can connect different proinsulin molecules together. This can create clumps/interconnected long chains of deformed proinsulin (termed “aggregates)”, which can be harmful to the cells (Figure 1). While the misfolding of proinsulin has been observed under various conditions ^{11, 13}, there were no methods available to measure the extent of folding or misfolding.

Additional notes: Two misfolded proinsulin single units (monomers) can connect via disulfide bonds and make a “dimer”, and when there are three misfolded proinsulins, they form a “trimer”, and so on to result eventually in large disulfide-liked proinsulin aggregates (Figure 1).
The two primary findings from the study (Protein Sci. 2024 Apr; 33(4): e4949) are summarized below. One outlines the technique used by the researchers to quantify misfolding, while the other underscores a novel discovery in diabetes research.

Key findings from the study:
1) The researchers developed a new method to measure the amount of incorrectly folded proinsulin in beta cells. Using a slightly modified version of a technique called immunoblotting (or western blotting), they were able to successfully quantify the amount of correctly folded and incorrectly folded proinsulin in both healthy and diabetic conditions. Using this approach, the authors also emphasized that the non-native proinsulin single units/monomers were the precursors to dimers and trimers (shown in Figure 1, right panel).

2) Research in the past two decades has pointed out that either faulty insulin genes or unhealthy conditions within the cell's quality control department (the ER) leads to severe proinsulin folding and trafficking problems. While it was previously thought that incorrectly folded proinsulin always causes problems with transporting proinsulin out of the ER, this study suggests that the opposite can also happen: problems with transporting proinsulin can actually cause it to fold incorrectly. This finding is important because it helps us better understand the causes of different types of diabetes, including neonatal diabetes, which affects babies and young children. Figure 2 shows how proinsulin folding is different in cells with normal and defective proinsulin transport.

Fixing the problem: Based on the second observation (above), the researchers thought that if they could fix the problem with transporting proinsulin out of the ER, it might also fix the problem with proinsulin folding incorrectly. When they tested this idea, they found that helping proinsulin travel out of the ER more efficiently did indeed help fix the problem of incorrectly folded proinsulin. This is the first time anyone has shown that incorrectly folded proinsulin can be fixed.

Conclusions and prospects: Proinsulin misfolding and impaired ER-to-Golgi trafficking are mutually reinforcing events (Figure 3), with each contributing to the other.


By understanding this relationship, researchers may be able to develop new strategies to prevent or treat diabetes. Understanding the root causes of this disease can lead to better ways to manage it and improve the lives of millions of people.